Radio Communication System, Base Station Apparatus, and Radio Terminal

1. A radio communication system comprising: a radio access network including a first base station configured to manage a first cell and a second base station configured to manage a second cell; and a radio terminal configured to support dual connectivity using the first cell and the second cell, the dual connectivity involving a split bearer that is a bearer split over the first base station and the second base station, wherein: the radio access network is configured to transmit, in the first cell, an initial Radio Resource Control (RRC) Connection Reconfiguration message, the initial RRC Connection Reconfiguration message including first Data Radio Bearer (DRB) configuration information for the first cell and second DRB configuration information for the second cell, the first DRB configuration information includes a first DRB Identity allocated to a first part of the split bearer in the first cell, the second DRB configuration information includes a second DRB Identity allocated to a second part of the split bearer in the second cell, the first DRB Identity is set to the same identity as the second DRB Identity, the transmitting of the initial RRC Connection Reconfiguration message follows a procedure for the radio terminal to transit from an RRC_IDLE state to an RRC_CONNECTED state, and a further procedure for the first base station to send to the second base station a request including the first DRB Identity for the dual connectivity and receive from the second base station an acknowledgement in response to the request, and the radio terminal is configured to recognize whether the bearer split is needed based on whether the initial RRC Connection Reconfiguration message includes security information to be used for the bearer split.

2. A first base station configured to manage a first cell, the base station comprising: a memory; and at least one hardware processer coupled to the memory and configured to: control dual connectivity using the first cell and a second cell managed by a second base station, the dual connectivity involving a split bearer that is a bearer split over the first base station and the second base station; and a transmitter configured to transmit, in the first cell, an initial Radio Resource Control (RRC) Connection Reconfiguration message, the initial RRC Connection Reconfiguration message including first Data Radio Bearer (DRB) configuration information for the first cell and second DRB configuration information for the second cell, wherein: the first DRB configuration information includes a first DRB Identity allocated to a first part of the split bearer in the first cell, the second DRB configuration information includes a second DRB Identity allocated to a second part of the split bearer in the second cell, the first DRB Identity is set to the same identity as the second DRB Identity, the transmitting of the initial RRC Connection Reconfiguration message follows a procedure for a radio terminal to transit from an RRC_IDLE state to an RRC_CONNECTED state, and a further procedure for the first base station to send to the second base station a request including the first DRB Identity for the dual connectivity and receive from the second base station an acknowledgement in response to the request, and the initial RRC Connection Reconfiguration message causes the radio terminal to recognize whether the bearer split is needed based on whether the initial RRC Connection Reconfiguration message includes security information to be used for the bearer split.

3. The first base station according to claim 2 , wherein in the split bearer, a first DRB in the first cell and a second DRB in the second cell are both mapped to a network bearer, and the network bearer includes at least one of E-UTRAN Radio Access Bearer (E-RAB) and Evolved Packet System (EPS) bearer.

4. A radio terminal comprising: a memory; and at least one hardware processor coupled to the memory and configured to perform dual connectivity using a first cell managed by a first base station and a second cell managed by a second base station, the dual connectivity involving a split bearer that is a bearer split over the first base station and the second base station; and a receiver configured to receive, in the first cell, an initial Radio Resource Control (RRC) Connection Reconfiguration message, the initial RRC Connection Reconfiguration message including first Data Radio Bearer (DRB) configuration information for the first cell and second DRB configuration information for the second cell, wherein: the first DRB configuration information includes a first DRB Identity allocated to a first part of the split bearer in the first cell, the second DRB configuration information includes a second DRB Identity allocated to a second part of the split bearer in the second cell, the first DRB Identity is set to the same identity as the second DRB Identity, the receiving of the initial RRC Connection Reconfiguration message follows a procedure for the radio terminal to transit from an RRC_IDLE state to an RRC_CONNECTED state, and a further procedure for the first base station to send to the second base station a request including the first DRB Identity for the dual connectivity and receive from the second base station an acknowledgement in response to the request, and the at least one hardware processor is configured to recognize whether the bearer split is needed based on whether the initial RRC Connection Reconfiguration message includes security information to be used for the bearer split.

5. The radio terminal according to claim 4 , wherein in the split bearer, a first DRB in the first cell and a second DRB in the second cell are both mapped to a network bearer, and the network bearer includes at least one of E-UTRAN Radio Access Bearer (E-RAB) and Evolved Packet System (EPS) bearer.

6. A method for a first base station configured to manage a first cell, the method comprising: controlling dual connectivity using the first cell and a second cell managed by a second base station, the dual connectivity involving a split bearer that is a bearer split over the first base station and the second base station; and transmitting, in the first cell, an initial Radio Resource Control (RRC) Connection Reconfiguration message, the initial RRC Connection Reconfiguration message including first Data Radio Bearer (DRB) configuration information for the first cell and second DRB configuration information for the second cell, wherein: the first DRB configuration information includes a first DRB Identity allocated to a first part of the split bearer in the first cell, the second DRB configuration information includes a second DRB Identity allocated to a second part of the split bearer in the second cell, the first DRB Identity is set to the same identity as the second DRB Identity, the transmitting of the initial RRC Connection Reconfiguration message follows a procedure for a radio terminal to transit from an RRC_IDLE state to an RRC_CONNECTED state, and a further procedure for the first base station to send to the second base station a request including the first DRB Identity for the dual connectivity and receive from the second base station an acknowledgement in response to the request, and the initial RRC Connection Reconfiguration message causes the radio terminal to recognize whether the bearer split is needed based on whether the initial RRC Connection Reconfiguration message includes security information to be used for the bearer split.

7. The method according to claim 6 , wherein in the split bearer, a first DRB in the first cell and a second DRB in the second cell are both mapped to a network bearer, and the network bearer includes at least one of E-UTRAN Radio Access Bearer (E-RAB) and Evolved Packet System (EPS) bearer.

8. A method for a radio terminal, the method comprising: performing dual connectivity using a first cell managed by a first base station and a second cell managed by a second base station, the dual connectivity involving a split bearer that is a bearer split over the first base station and the second base station; and receiving, in the first cell, an initial Radio Resource Control (RRC) Connection Reconfiguration message, the initial RRC Connection Reconfiguration message including first Data Radio Bearer (DRB) configuration information for the first cell and second DRB configuration information for the second cell, wherein: the first DRB configuration information includes a first DRB Identity allocated to a first part of the split bearer in the first cell, the second DRB configuration information includes a second DRB Identity allocated to a second part of the split bearer in the second cell, the first DRB Identity is set to the same identity as the second DRB Identity, the receiving of the initial RRC Connection Reconfiguration message follows a procedure for the radio terminal to transit from an RRC_IDLE state to an RRC_CONNECTED state, and a further procedure for the first base station to send to the second base station a request including the first DRB Identity for the dual connectivity and receive from the second base station an acknowledgement in response to the request, and the method further comprises recognizing whether the bearer split is needed based on whether the initial RRC Connection Reconfiguration message includes security information to be used for the bearer split.

9. The method according to claim 8 , wherein in the split bearer, a first DRB in the first cell and a second DRB in the second cell are both mapped to a network bearer, and the network bearer includes at least one of E-UTRAN Radio Access Bearer (E-RAB) and Evolved Packet System (EPS) bearer.

10. The radio communication system according to claim 1 , wherein the first base station is configured to use a first radio access technology, and the second base station is configured to use a second radio access technology different from the first radio access technology.

11. The first base station according to claim 2 , wherein the first base station is configured to use a first radio access technology different from a second radio access technology which the second base station uses.

12. The radio communication system according to claim 1 , wherein the first base station is configured to send, to a core network, an indication for establishing the split bearer with the core network.

13. The first base station according to claim 2 , wherein the first base station is configured to send, to a core network, an indication for establishing the split bearer with the core network.

14. The method according to claim 6 , further comprising: sending, to a core network, an indication for establishing the split bearer with the core network.